Seattle's tunnel boring machine that's been digging beneath the city for the State Route 99 tunnel reached the end of its 1.75-mile journey Tuesday, nearly four years after it embarked on the job.

Just before 10 a.m., Bertha pierced the cement wall of the tunnel, sending water, dirt and a plume of dust into the disassembly pit. About an hour and a half later, Bertha's cutterhead emerged from the earth in a huge plume of dust and steam.

Seattle Tunnel Partners told neighbors dust was possible during the breakthrough, and the dust is not toxic, according to WSDOT. The concrete barrier was reinforced with fiberglass, but most of the dust came from the concrete.

If anyone has concerns about the dust they can contact Seattle Tunnel Partners at 1-888-AWV-LINE.

The event marks the end of Bertha's 9,270 foot journey that she began July 2013. But the tunnel still has awhile to go before it will open to the public. Crews need to finish the double-decker lanes and other infrastructure.

Seattle Mayor Ed Murray says completing a tunnel that will serve as an underground highway was "a major construction milestone in our plan to reclaim Seattle's waterfront." He added the city is one step closer to taking down the viaduct for "our vision of a well-connected 21st century city" serving "pedestrians, transit and sensible car trips."

Washington Governor Jay Inslee says the breakthrough is "a historic moment in our state's transportation history." But critics call it an expensive vanity project, and environmentalists have objected to building another highway in Seattle.

The tunnel project was not always a smooth journey. From a sinkhole that shut down operations for months to cost overruns that could climb to $149 million, the project caused some headaches.

Former Washington Governor Christine Gregoire, who was a driving force to replace the Alaskan Way Viaduct with a tunnel, said she always believed it was the right decision.

"Even during the worst of times, I never lost faith," Gregoire said. "That doesn't say I wasn't praying it would happen, but I've always been optimistic it was going to get done."

Gregoire believes the tunnel had to be built and points to studies that the viaduct likely wouldn't survive another earthquake.

Meanwhile, some lawmakers are working in Olympia on a bill to make Seattle pay for the cost overruns on the tunnel instead of the state of Washington.

Republican Representative Ed Orcutt says back when Seattle Mayor Ed Murray was in the legislature, he included language in the project, promising that Seattle property owners would pay for any overruns, either through a property or sales tax.

KING 5's Liza Javier, Allison Sundell and the Associated Press contributed to this report.